In the field of mass analysis using an ion trap mass spectrometer or other apparatuses, a technique called the MS/MS analysis (or tandem analysis) is conventionally known. In a general MS/MS analysis, an ion having a specific mass (or mass-to-charge ratio, m/z, to be exact) is first selected as a precursor ion from an object to be analyzed. Next, the selected precursor ion is dissociated by a collision induced dissociation (CID) process to produce product ions (also called fragment ions). The resulting product ions are subjected to a mass analysis to obtain information relating to the mass of the product ions, the ions and neutral molecules desorbed by the dissociation operation, and other particles. Based on this information, the composition and chemical structure of the target sample molecule are deduced.
In recent years, samples to be analyzed with this type of system have been progressively increasing in molecular weight and becoming more complex in structure (or the composition). Therefore, depending on the nature of the sample, it is possible that the sample cannot be dissociated into sufficiently small masses by only one stage of the dissociation process. In such a case, an MSn analysis may be performed, where the dissociation operation is repeated two or more times and the eventually obtained product ions are subjected to mass analysis (for example, refer to Patent Document 1, 2 or other documents). The aforementioned MS/MS analysis is an MSn analysis where n=2.
In general, mass spectrometers create a mass spectrum (MSn spectrum), with the horizontal axis indicating the mass-to-charge ratio and the vertical axis indicating the signal intensity (relative intensity), as the result of mass analysis and presents it on a display screen as one of the analysis results. In the case of a mass spectrometer capable of MSn analyses, a plurality of precursor ions having different masses can be respectively selected, in which case an MSn spectrum will be obtained for each precursor ion. An MSn spectrum provides various types of peak information reflecting the state of the molecular bonds of the original compound. Accordingly, a plurality of compounds having similar structures are likely to show MSn spectrums having similar patterns.
By the way, analyzing metabolites resulting from chemical changes in a living organism is a crucial subject in many fields, such as the diagnosis of various kinds of diseases and illnesses, the assessment of the effectiveness and safety of drugs and functional foods, and the research on lifestyle and health. In recent years, a method called Metabolomics for exhaustively analyzing a metabolite has been attracting attention. In this metabolite analysis, the aforementioned method using MSn spectrums is useful to search for a compound resulting from a metabolism of another compound having a known structure (this compound will be hereinafter called a “parent compound”, and the former compound will be called a “metabolite”). This is due to the fact that a metabolite results from a partial modification in the structure of a parent compound and their MSn spectrums include many common features. Accordingly, by comparing their MSn spectrums, it is possible to extract candidates for the metabolite from a large number of compounds. Software programs for automatically performing the analysis process described to this point have been already provided.
However, to achieve the ultimate objective, i.e. the deduction and determination of the structure of a metabolite, an analysis operator needs to visually check MSn analysis data and other data and make a judgment. Improving the efficiency of this task has been a major challenge to enhance the throughput of the analysis. One reason for the inefficiency of this checking task is that it is difficult to immediately, or intuitively, visually identify the peak that corresponds to the modified portion of the parent compound or metabolite in the MSn spectrum. Another reason is that, even when it is appropriate to increase the number of stages of the dissociation operation (i.e. to perform the MSn analysis with a large value of n), it is not easy to decide which peak should be given priority to be the precursor ion for the next stage.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-142196
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-249114